Muon Collider Physics

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Muon Collider Detector Backgrounds
Fermilab
Accelerator Physics Center
Nikolai Mokhov

• Muon Collider Physics Detectors Workshop
• Fermilab
• March 5, 2008

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OUTLINE

• Introduction and a Little History
• Three Background Sources
• Detector Performance and Tolerable Limits
• IP Backgrounds
• Muon Beam Decay Backgrounds
• Muon Beam Halo
• Summary

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INTRODUCTION

• The high physics potential of a Muon Collider
  (MC) is reached only if a high luminosity of mm-
  collisions in the TeV range is achieved (gt1034
  cm-2 s-1). The overall detector performance in
  this domain is strongly dependent on the
  background particle rates in various
  sub-detectors. The deleterious effects of the
  background and radiation environment produced by
  the beam in the ring are very important issues in
  the Interaction Region (IR) and detector design.

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REFERENCES ON MUON COLLIDER BACKGROUNDS
1. G.W. Foster and N.V. Mokhov, "Backgrounds and
Detector Performance at 2x2 TeV mumu- Collider",
Sausalito-94, AIP Conf. Proc. 352, pp. 178-190.
Fermilab-Conf-95/037 (1995). 2. N.M. Gelfand and
N.V. Mokhov, "2x2 TeV mumu- Collider Lattice
and Accelerator-Detector Interface Study", Proc.
of PAC05. Also Fermilab-Conf-95/100 (1995). 3.
N.V. Mokhov and S.I. Striganov, "Simulation of
Backgrounds in Detectors and Energy Deposition in
Superconducting Magnets at mumu- Colliders",
Proc. of 9th ICFA Workshop, Montauk, NY, October
15-20, 1995. Also Fermilab-Conf-96/011
(1996). 4. N.V. Mokhov, "Comparison of
Backgrounds in Detectors for LHC, NLC and mumu-
Colliders", Nucl. Phys. B, 51A (1996) pp.
210-218. 5. C.J. Johnstone and N.V. Mokhov,
Optimization of a Muon Collider Interaction
Region with Respect to Detector Backgrounds and
the Heat Load to the Cryogenic Systems,
Fermilab-Conf-96-366 (1996). 6. C. Ankenbrandt
et al., Status of Muon Collider Research and
Development and Future Plans, Phys. Rev. ST-AB,
vol. 2, 081001 (1999) pp. 1-73. 7. Snowmass 1996
Feasibility Study. 8. Muon Collider workshops,
1995-1997 (B. Foster, C. Johnstone, N. Mokhov, I.
Stumer)

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BACKGROUNDS

• Three sources at Muon Collider
• IP backgrounds Particles originated at
  interaction point (IP) from mm- collisions as
  well as incoherent pairs.
• Muon beam decay backgrounds Unavoidable
  bilateral detector irradiation by particle fluxes
  from the beamline components and accelerator
  tunnel major source at MC.
• Beam halo Beam loss at limiting apertures
  unavoidable, but is taken care with an
  appropriate collimation system far upstream of IP.

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DETECTOR PERFORMANCE

• Backgrounds affect collider detector performance
• in three major ways
• Detector component radiation aging and damage.
• Reconstruction of background objects (e.g.,
  tracks) not related to products of mm-
  collisions.
• Deterioration of detector resolution (e.g., jets
  energy resolution due to extra energy from
  background hits).

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Strawman MC Detector Concept (1)

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Strawman MC Detector Concept (2)

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BACKGROUND TOLERABLE LIMITS

Calorimeter, tracker and vertex detectors in
smallest element, occupancy 1. To avoid
pattern recognition problem in tracker, hit
density from charged particles should be 0.2
hit/cm2/bunch. Muon system the RPCs (sensitive
media) need 1 ms to re-charge a 1 cm2 area around
the avalanche, therefore, the hit rate in excess
of 100 Hz/cm2 would result in an unmanageable
dead time. With 80 sensitive layers in a Muon
Endcap, it corresponds to a muon flux at its
entrance of about 1 m/cm2/s.
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TEMPORAL ASPECTS

• Temporal considerations in the IP and machine
  background analysis are of a primary importance.
  Integrated levels determine radiation damage,
  aging and radio-activation of detector components
  as well as the radiation environment in the
  experimental hall, accelerator tunnel and their
  surroundings. High instantaneous particle fluxes
  complicate track reconstruction, cause increased
  trigger rates and affect detector occupancy.
• One can define the instantaneous or effective
  luminosity - which determines the detector
  performance for the amount of radiation in the
  detector active element over the
  drifting/integration time ?td (sensitivity
  window) or the bunch train length, whichever is
  smaller. For detector elements most susceptible
  to occupancy problem ?td is 40 - 300 ns.

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IP Backgrounds at MC vs LHC and ILC

MC produces 3e-7 of LHCs background hadrons from
IP annually, while instantaneous background rate
is 0.025 of the LHC one. Incoherent pair
production might be a concern!
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Muon Beam Decays Major Source of Backgrounds
Contrary to LHC, almost 100 of background and
radiation problems at MC arise in the lattice.
Muon decays is the major source. For example, the
decay length for 2-TeV muons is lD 107 m. With
2e12 muons in a bunch one has 2e5 decays per
meter of the lattice in a single pass, 2e8 decays
per meter per store, and 6e9 decays per meter per
second. Electrons from muon decay have mean
energy of approximately 1/3 of that of the muons.
At 2 TeV, these 700-GeV electrons, generated at
the 6e9 m-1 s-1 rate, travel to the inside of the
ring magnets, and radiate a lot of energetic
synchrotron photons towards the outside of the
ring. Electromagnetic showers induced by these
electrons and photons in the collider components
generate intense fluxes of muons, hadrons and
daughter electrons and photons, which create high
background and radiation levels both in a
detector and in the storage ring. The primary
concern is muon decays in the inner triplet and
near IP.

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Collimating Nozzles at IP
Due to the very high energy of electrons and
photons in the large aperture, the whole triplet
is a source of backgrounds in the detector. As
calculated, electron and photon fluxes and energy
deposition density in detector components are
well beyond current technological capabilities if
one applies no measures to bring these levels
down. As was found, the most effective
collimation includes a limiting aperture about
one meter from the IP, with an interior conical
surface which opens outward as it approaches the
IP. These collimators have the aspect of two
nozzles spraying electromagnetic fire at each
other, with the charged component of the showers
being confined radially by the solenoidal
magnetic field and the photons from one nozzle
being trapped (to whatever degree possible) by
the conical opening in the opposing nozzle.

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IP Region

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Nozzle Concepts
R4cm

Background reduction 30 to 500 times
Detector is not connected by a straight line
with any surface hit by decay electrons in
forward or backward directions
4m
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Spreading Decay Electrons Along Final Focus
SC sweep dipoles with tungsten collimators
between elements implemented into inner triplet
another factor of seven background reduction

Even with all of the above, muons are orders of
magnitude above the limit
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Muon Collider IR

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3mm ? IR and CCS optics, 2x2 TeV Collider
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Occupancy for 0.3 x 0.3 mm Si-pads

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Radiation Damage to Si

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Incoherent Pairs Nozzles Solenoid Field

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Bethe-Heitler Muons

2x2 TeV R4m L130m from IP

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Bethe-Heitler Muons

Significant fluctuations in transverse energy and
missing transverse energy due to energy spikes in
deep inelastic interactions of BH muons BH muon
and accompanying particle fluxes substantially
exceed tolerable limits, in particular in muon
system ILC-type magnetic shielding
walls upstream of IR are needed !!!
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MUON BEAM HALO
We have shown that detector backgrounds
originating from beam halo can exceed those from
decays in the vicinity of IP. Only with a
dedicated beam cleaning system far enough from IP
can one mitigate this problem.

Tracker
Muons injected with large momentum errors or
betatron oscillations will be lost within the
first few turns. After that, with active
scraping, the beam halo generated through
beam-gas scattering, resonances and beam-beam
interactions at the IP reaches equilibrium and
beam losses remain constant throughout the rest
of the cycle.
Endcap Calor.
Particle fluxes in detector for 2-TeV beam beam
halo loss (1 per store) at 200m from IP
Without scraping orders of magnitude above the
limit
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SCRAPING MUON BEAM HALO

• We have designed two beam scraping systems to
  (completely) suppress beam halo contribution to
  detector backgrounds
• For TeV domain, extraction of beam halo with
  electrostatic deflector reduces loss rate in IR
  by three orders of magnitude efficiency of an
  absorber-based system is much-much lower.
• For 50-GeV muon beam, a five meter long steel
  absorber does an excellent job, eliminating
  halo-induced backgrounds in detectors.

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SUMMARY (1)

• Backgrounds originated at IP are negligible
  compared to other sources hadrons from mm-
  collisions incoherent pairs are captured by
  nozzles in the solenoid field.
• Backgrounds induced by beam halo losses exceed
  the limits by orders of magnitude, but can be
  suppressed with an appropriate collimation
  system.
• Muon beam decays are the major source of
  backgrounds in the MC detectors. They can
  drastically be reduced by sophisticated
  collimating nozzles at IP, and sweep dipoles and
  collimators in a 100-m region upstream IP.

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SUMMARY (2)

Still work to do

• With current design, total occupancy is OK for
  0.5-TeV MC, 2-3 times above the limit for 4-TeV
  MC, and up to 10 times above the limit at r lt 15
  cm for 0.1-TeV MC.
• Bethe-Heitler muons in calorimeter and forward
  muon system are a few orders of magnitude above
  the limit ILC-type magnetic shielding walls
  upstream IR needed.